There is a short YouTube video of an 350D operator holding the top right button down and entering “Service Menu” mode. You don’t start the machine, just turn the key on. Easy once you figure it out. Unfortunately, unless it’s throwing an active code at that moment, they will all read normal. And if if does have an active code, you will have to have a way to decode it. Basically, service mode is pretty well worthless.
A JD mechanic told me the G series is much better. I would hope so. I love the control panel of my JD K dozers, they tell you the code and what it means and when it did it, etc. Excavators should follow suit.